AbstractThe evolution of dioecy was studied inSchiedea(Caryophyllaceae), a genus endemic to the Hawaiian Islands. Eight of the 22 species are diclinous, possessing gynodioecious, subdioecious, or dioecious breeding systems. A biogeographic analysis of the genus indicates that the ancestor ofSchiedeacolonized early in the history of the Hawaiian Islands. Subsequently, hermaphroditic species appear to have engaged in inter‐island colonization more frequently than diclinous species. For this reason, single‐island endemism and dicliny are more common on the older Hawaiian Islands. Strong inbreeding depression was detected in three species ofSchiedea, indicating that genetic factors have played a role in the evolution of dicliny. Depending on the level of natural selfing, the expression of inbreeding depressioin may have favored the outcrossed progeny of rare females in populations, and eventually the evolution of dioecy. In contrast to evidence for inbreeding depression, there was very little evidence that resource allocation, sex lability, or habitat partitioning have played an important role in the evolution of dioecy. In subdioeciousS. globosahermaphrodites were largely male in function, and in gynodioeciousS. salicariafemales and hermaphrodites were equivalent in nearly all aspects of female function that could be measured. Variation in breeding systems inSchiedeaand the closely relatedAlsinidendronmay result from the past history of population bottlenecks that have resulted in varying levels of inbreeding depress